NOTE: Photon Trigger files should only be changed
by, or with direct authorization of, the run coordinator.

1 Introduction

During CLAS photon beam runs, level 1 [L1] triggers
are produced by the coincidence of the so-called "photon triggers" and
"CLAS triggers. The CLAS trigger is a logic combination of CLAS detector
signals made by the Tigris software and it is note addressed in
this document. . The photon trigger is and input to the trigger supervisor
[TS]. This note documents the hardware and software needed to set the photon
trigger. There is hardware to make possible two photon trigger inputs into
the TS, they are called ASYNC1 and ASYNC2.

The photon trigger provides to the TS the following
information:

The tagger "master or" (coincidence of all or sub-sets
of the tagger’s T counter signals) [MOR]. The MOR can include all of the
T counters or any sub-set (i.e., to select a specific energy range). This
selection needs to be done manually in hardware (see section 2).

A logic combination of start-counter signals [ST]. The
are three ST signals covering sets of two CLAS sectors (1-2, 3-4, 5-6).
A logic combination of the three ST signals (one of three [1/3], two of
three [2/3] or all [3/3]) needs to be done manually in hardware
(see section 2).

Total absorption counter signal [TAC]. The sum of the
four TAC detector signals is available for triggering.

Pair Spectrometer signal [PS].

Any logical combination of MOR, ST, TAC or PS

2 Hardware

The hardware for the photon trigger is located in
part at the space frame [SF] (1st floor), and in part at the
forward carriage [FC] (2nd floor).

Space frame hardware

The MOR and logic ST signals are defined in the
SF hardware.

[MOR]The MOR is produced by a series of "cascading" coincidences
done by NIM Phillips 754 modules. See figure 1 for the general tagger system
electronic diagram. These modules are to the left of the tagger electronic
racks, entering the Hall to your right. There are "extra" 754 modules,
more than the necessary to make just one combination of all T counters.
Therefore, up to TWO MORs can be defined as any subset of T counter ranges.
There are two cables of same length (about 110ns) running from the SF to
the FC electronics carrying these two MOR signals for ASYNC1 and ASYNC2.

G6C: two equal MOR are defined by the 1-12 T counters
Two 754 modules are used to combined [1-4,5-8,9-12].MOR for ASYNC1 and ASYNC2 are the same in g6c.

[ST]The ST logical signals are made in the rack to the
right of where the MOR signals are defined. There is the possibility for
two ST combinations going to the FC (again, there are two equal length
cables – also of the same length as the MOR cables). They will be combined
with the MOR signals to form ASYNC1 and ASYNC2 at the FC. There are six
ST signals coming to the rack. 1-2, 3-4 and 5-6 are added and sent to a
NIM 740 Phillips module. See diagram in figure 2.

G6C: ASYNC1 is the logic signal of 2 out of three
(2/3) hits.ASYNC2 is the logic signal of 1 out of three
(1/3) hits.

Forward Carriage

The discrimination, relative delays and logical
handling of the MOR, ST and TAC (PS) signals are made at the FC through
CAMAC modules (located in crate #2). The modules are at CAMAC1, top of
rack that contains the TS.

MoModule

slot

description

ASYNC1

4413

3

MOR discriminator (test input
behind)

4413

5

ST discriminator (test input
behind)

4418

4

ST delay (MOR is currently
not delay)

4516

9

MOR ST logic AND

ASYNC2

2313

20

MOR discriminator (input
at address 0)

2313

21

ST discriminator (input at
address 8)

4418

22

ST delay (MOR is currently
not delay )

4516

23

MOR ST logic AND

The photon trigger software (see section 3) controls
the values (widths, thresholds and logic sets) for these units. Modules
4413 allow only threshold control, however, widths and thresholds can be
control for the 2313 units. There are two photon triggers made in parallel.
Normally, Both will make use of a MOR·
ST coincidence with adjustable widths, thresholds and relative delays.ASYNC1: The modules for the ASYNC1 trigger
are to the left of the CAMAC unit (crate 2). The MOR signal from
the SF enters a 4413 discriminator unit (from the test input, therefore,
the signal is available in all 16 channels). Input 1 goes directly to the
logic module (i.e. no delay) (level 1) and input 16 goes directly to bit
7 of TS. The ST signal enters another 4413 discriminator unit but also
through the test input. The ST signal goes through a 4418-delay unit where
the relative timing is adjusted (during commissioning in steps of 2 ns
from 2 to 32 ns).The TAC signal enters a 2313 discriminator (no currently
in use).

ASYNC2: The modules for the ASYNC2 trigger
are to the right of the CAMAC unit (crate 2). The MOR signal from
the SF enters a 2313 discriminator unit (from the input #1 (address 0).
Input 1 goes directly to the logic module (no delay) (level 1). The ST
signal enters another 2313 discriminator unit but through channel #9 (address
8). The ST signal goes through a 4418-delay unit where the relative timing
is adjusted (during commissioning in steps of 2 ns from 2 to 32 ns).There is not TAC signal related to ASYNC2.

3 Software

The FC CAMAC electronics is controlled from the CLON
machines using the photon gui or editing and setting the
photon trigger files "by hand".

The photon gui function is an interface to produce
the photon trigger file. The trigger configuration from the DAQ run the
setting of the photon trigger electronics as specified by this file at
the initiation of each experimental run.

["by hand"]Files related to the photon triggers are at:

$CLON_PARMS/photon

for g6C the file is:

$CLON_PARMS/photon/g6/config/phtrigger_asybc12_startm.pl

that file can be set "manually" using the command:
(from the above directory):

photon_setup camac1 phtrigger_asybc12_startm.pl
check

4 Commissioning the photon trigger

Timing MOR to ST

A "time curve" using the ST delay modules need
to be performed during commissioning (or any change of hardware configuration
that could affect the MOR-ST timing relation) to find the optimum MOR-ST
coincidence timing.

One way to make this curve is using the information on the TLV1 banks

Use: /home/clasrun/tlv1/bin/tlv1

5 Production Running with the photon
trigger

The "photon trigger file" setting the electronics
in the FC is controlled by the DAQ configuration file. It is run at the
initialization of each run. Normally, it should not be change after commissioning
– consult the run coordinator to make any changes.